[ci] #118: Add tests jobs for Python from v3.6 to v3.9

- Extend test job by changing PYTHON_VERSION
- Splitted among two runners
- Rename stage

.gitlab-ci.yml

 stages:
   - format
-  - test
+  - tests
   - build
   - release
   - publish_doc
@@ -48,18 +48,40 @@ format:
   script:
     - make check-format
 
-test:
-  extends:
-    - .changes
-  stage: test
+.tests:
+  extends: .changes
+  stage: tests
+  image: $DOCKER_IMAGE/$PYTHON_VERSION:latest
   script:
     - poetry install --no-dev
     - make tests
 
+tests-3.6:
+  extends: .tests
+  tags: [poetry-56]
+  variables:
+    PYTHON_VERSION: "3.6"
+
+tests-3.7:
+  extends: .tests
+  tags: [poetry-78]
+  variables:
+    PYTHON_VERSION: "3.7"
+
+tests-3.8:
+  extends: .tests
+  tags: [poetry-78]
+    PYTHON_VERSION: "3.8"
+
+tests-3.9:
+  extends: .tests
+  tags: [poetry-56]
+  variables:
+
 check:
   extends:
     - .changes
-  stage: test
+  stage: tests
   script:
     - poetry install
     - make mypy